High frequency systems, including radio frequency (RF) and microwave systems, generally comprise multiple high frequency integrated circuits (IC) and discrete devices interconnected on a printed circuit board. Typically, a high frequency system requires generation and amplification of a distortion free fundamental operating frequency. As a practical matter, high frequency components and systems often generate unwanted harmonics of the fundamental operating frequency through amplification and switching functions conventionally performed within the system. It is desirable to filter out these harmonics generated by the system so that the signal with which the system operates is as free of harmonic distortion as possible. A low harmonic distortion system generally exhibits better efficiency overall. In order to reduce harmonic distortion, it is conventional to filter out harmonics of the fundamental frequency at an RF output port on one or more of the ICs within the system. It is a matter of designer judgment at which IC or ICs to place the filter.
An article entitled "Minimizing GSM Mobile-Terminal Design Risk Allows Easier Upgrades" by Kalinka & Baker from the May 1996 publication of Wireless Systems Design magazine, shows conventional filtering of harmonic distortion at both the input of a power amplifier (PA) and the output of a low noise amplifier (LNA)/RF mixer in a GSM terminal. The filters shown by way of block diagram representation typically comprise from three to seven discrete elements or ceramic resonators soldered to the printed circuit board and connected to an RF port lead of the IC. Disadvantageously, these filters can be complex and generally consume a relatively large amount of usable surface area on the printed wiring board. Additionally these filters can contribute up to 1 dB of insertion loss to the system. In the case of a power amplifier IC used with conventional filtering, in order to achieve the desired system output power, the power amplifier must output sufficient power to supply the output power required by the system at the output stage of the filter in addition to the power lost due to the insertion loss of the filter. This additional output power requires more DC current be drawn by the power amplifier. The added DC current drain disadvantageously decreases efficiency and increases heat generation. Accordingly, the traditional filtering contributes to a larger, more complex, and less efficient system design which is in contravention of the interest to miniaturize and to extend battery life.
There is a need, therefore, for improved filtering in a high frequency system that does not adversely effect insertion loss, efficiency or miniaturization.